Vehicles, whether for the transportation of humans or otherwise, may lack stability either by design, where inherent stability is sacrificed in order to achieve relative mobility, or due to the nature of their use. Many such vehicles are particularly susceptible to tipping, whether due to collision, mechanical failure, sudden turns, steep inclines, or an encounter with a surface irregularity for which the mechanism is incapable of compensating. Indeed, any personal vehicle may tip when sufficiently severe surface irregularities are encountered. Under these circumstances, the occupant or contents of the vehicle must be protected so that injury does not result from propulsion of the occupant toward the ground or other solid obstacle. Unless protected, the occupant may also sustain injury due to trauma or crushing if the vehicle, which may be relatively massive, overturns or is propelled, due to its inertia, into a solid obstacle or on top of the occupant. To protect the occupant of the vehicle from injury and the vehicle from damage, the vehicle must be inhibited from tipping over.
One method which has been employed in a wheelchair in order to prevent tipping under conditions of a sudden stop or sudden acceleration involves the placement of a roller or wheel forward of the front axle or aft of the rear axle. The purpose of the fixed roller or wheel is to contact the ground and roll in case the vehicle tips forwards or backwards, thereby inhibiting the vehicle from overturning. The presence of a fixed wheel extended forward of the frame of the vehicle limits the maneuverability of the vehicle by reducing ground clearance and restricting the vehicle's ability to traverse irregular terrain and obstacles. Limited maneuverability is particularly detrimental if the vehicle is a wheelchair. A similar limitation is imposed by a fixed wheel located aft of the rear axle to inhibit backward tilting of the vehicle.
Another anti-tip mechanism which has been applied to a wheelchair in order to prevent rearward tipping involves a pair of wheels mounted aft of the rear axle beneath the undercarriage of the wheelchair on spring-tensioned articulated supports.